To be worried about cost vs fairness for DC2 education

Whilst I agree ‘fair’ doesn’t always mean ‘equal’, in this case I think sending one to a private school vs State is too big a disparity to justify under this argument. It isn’t just the benefits that a private school can have for SEN children, but the friends he will be exposed too, the role models (from teachers to elder brothers and sisters of other students), and the opportunities. These are day-to-day small things that have measurable impact on your child’s personality, the thinking they are exposed to, and then the life choices they will then make. These will have a far greater impact than cost cutting measures, like an annual family holiday overseas vs in the UK.

If it was a tutor for one child and not another, or tennis lessons for a talented child and not another (or vice versa), I’d say fine, as that is a smaller thing. But I think the difference in schools is too big a difference to justify.

That's such a one dimensional way of thinking. My SEN son came out of private with some but not many qualifications. In paper you could say it wasn't "worth it" but he had a good, safe place where he wasn't getting his head kicked in every other week as was happening at the local school. It's not just about the ultimate grades.

You do what is right for each child. There is no such thing as an objectively "better" school..it's about the fit. Your younger child may be absolutely fine at the comp, or fine for KS3 but then move across for GCSEs. Fair does not mean identical.

Depends really - how bad is his SEN - is he likely to be able to lead a full adult life - get a job etc? is this investment in private schooling actually going to be worth it in adulthood?

It’s not something I would feel comfortable with as you don’t know how it will play out. If the dc who goes to state ends up fine then it’s fine but there is often resentment which i’ve seen in real life if they end up in different places if that makes sense.

Startlooking into a plan if it turns out DC2 isn't "fine"... or just to extend them. Tutoring, extra curricular, extra opportunities etc.

It was many, many years ago now, but my parents scrimped, saved and worked extra hours etc to send my elder brother (undiagnosed issues, but clearly not neuro-typical) to a private school. It didn't really work out, and he left with poor qualifications, and his issues persist (he's a pensioner now, btw, this was so long ago).
I went to state school, and personally I was fine, and thrived well in that system,despite some (lesser) issues myself (I'm probably not neuro-typical either, but it's been an asset rather than a burden).
Even as a kid I could recognise what my parents were trying to do (make a level playing field), and I've never resented them for it. They did what they thought was best, and worked bloody hard to try to do their best for my brother, who needed a lot of extra support.

Remember that stretching to afford private for both also means forgoing other opportunities that might benefit your second child. You also will not be in as good of a position to help him with university costs or other expenses of transitioning to adulthood.

Fairness is not about cost. Your non-SEN child already has many advantages that the SEN-child doesn’t. All you are doing is evening up the playing field a bit. What’s more important than the money is making sure both kids get your time and attention .